1. Technical Field
The present invention pertains to an improved process for obtaining long-chain N,N-dimethyl-N-alkylamines by catalytic hydrogenation of the corresponding dimethylamides.
It involves obtaining by this route N,N-dimethylamines free of heavy alcohols and intended, among other applications, for the manufacture of quaternary ammonium salts for industrial detergents, bactericides or disinfectants.
2. Prior Technique
The procedure is known for producing N,N-dimethyl-N-alkylamines by reaction of the dimethylamine with an alkyl halogen, a fatty alcohol or an alpha-olefin. These are industrial routes, but they involve expensive raw materials.
A more economic route involves reacting the dimethylamine on a fatty acid to form the N,N-dimethylaklylamide, then hydrogenating the fatty N,N-dimethylalkylamide to the N,N-dimethyl-N-alkylamine. Many modes of industrial realization are known, which operate by catalytic hydrogenation under hydrogen pressure; for example:
(i) the procedure presented in U.S. Pat. No. 3,190,922 filed by General Mills operates on a copper chromite catalyst which, it is said incidentally, can be stabilized with barium oxide, in the presence of dimethylamine and under a circulating hydrogen stream. However, the catalysts used, whether or not they are stabilized with barium, turn out to lose too large a portion of their activity and their selectivity;
(ii) the procedure according to U.S. Pat. No. 3,444,204, which is a continuous procedure based on the use of catalysts of the same type, in a fixed bed under high pressure (250 bars);
(iii) the procedure described by L. Pashkova and M. Yakushkin in J. of Applied Chem. of USSR, 53, 8, pp. 1398-1401 (1980), which is distinguished from the preceding essentially in that one operates at atmospheric pressure; and
(iv) U.S. Pat. No. 4,448,998 of Procter and Gamble, which represents an attempt to improve the quality of the finished raw products, with use of a hydrogenation catalyst of copper chromite combined with a proportionate amount of non-catalytic zeolite (18% in relation to the charge), so as to capture the water produced during the hydrogenation of the amide and to minimize the parasitic reactions that it generates.
These known procedures of the prior art have in common the drawbacks of not providing fatty N,N-dimethyl-N-alkylamine of the desired quality for at least two reasons.
First, the purity of the final product is insufficient and, specifically, the content of fatty alcohols is too high. These alcohols are very difficult to separate out by distillation since they boil very close to the N,N-dimethyl-N-alkylamines and it involves not just separating a fatty alcohol from the homologous N,N-dimethyl-N-alkylamine, but separating a group of fatty alcohols from a group of N,N-dimethyl-N-alkylamines, the distribution of the chains of which is that of the fatty acids from the source oils, i.e., coconut oil, palm oil, etc. It is easy to comprehend this from the table below of the comparative boiling temperatures under 20 mm of mercury (2.7 kPa) for the N-N-dimethyl-N-alkylamines and the n-alkanols:
______________________________________ Length of alkyl chain 8C 10C 12C 14C 16C 18C N,N-dimethylalkylamine 71 112 140 169 191 215 Alkanol 101 126 150 177 198 220 ______________________________________
Second, the economically necessary recycling of the copper chromite catalyst leads to a decrease in its activity and accentuates its non-specificity, which is balanced in each cycle by an increase in the content of alcohols.